It's called baby steps. Implement the HTML extensions you want using JS, then get them standardized. You could do it w/o the PoC first, but then it might be harder to get the mindshare and traction you need to get the W3C to go with it.

Even if the W3C rejects this, it would still be useful for many who want to write declarative scripts rather than actual JS.

Why does CSS exist anyways? Because it's a declarative language, and declarative languages are clearly better for certain tasks. HTML itself is declarative.